It is increasingly recognised that chronic exposure to opioids has been associated with neuropsychological impairment during both active use and following a period of abstinence. The overall objective of this thesis was to review the relevant prior literature in a systematic manner and subsequently to describe the effects of chronic exposure to prescribed and illicit opioids using an ambispective cohort study design. A systematic literature review was conducted to identify if chronic (defined as a period for more than 3 months) exposure to opioids (prescribed and/or illicit) was associated with measurable neuropsychological deficits. This review was conducted accordingly to the Meta-analysis of Observational Studies in Epidemiology (MOOSE) and the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guidelines. The results were subsequently described within three cognitive domains of intelligence, executive function and memory and learning. Out of a total of 905 articles extracted between 1964 and 2009, 49 articles were considered appropriate for selection and review. Studies of current and abstinent chronic opioid users (illicit heroin users, patients prescribed methadone for illicit opioid dependence and patients taking opioids as part of the management of chronic pain) have identified performance deficits in measures of executive functioning and memory. These have included impairments within the domains of cognitive and motor impulsivity, strategic planning, cognitive flexibility, attention and memory. However other studies found no clear deficits when comparing the performance of healthy controls. The literature suggested that these neuropsychological deficits may be subject to at least partial recovery following initiation of methadone or total withdrawal from any opioids.This review also highlighted several methodological issues that affect the reliability, validity and clinical relevance of the results obtained. Subsequently a two year ambispective cohort design study was conducted which tested representative opioid exposed participants and healthy controls. Cohorts of participants with validated histories of illicit heroin use (HEROIN, n=24), stabilised methadone maintenance (METHADONE, n=29), chronic opioid prescriptions for pain (CHRONIC PAIN, n=28) and controls (HEALTHY CONTROL, n=28) were recruited. The study was designed to test neuropsychological performance in the HEALTHY CONTROL and CHRONIC PAIN groups on one occasion; and for the HEROIN and METHADONE groups on three and two occasions respectively. The intention was to describe neuropsychological performance in the HEROIN group under conditions of stable illicit heroin use, in controlled opioid withdrawal and when subsequently stabilised on methadone. For the METHADONE group, participants were tested twice, six months apart, to test for changes induced by chronic exposure to methadone. Eligible, screened and consented individuals were tested on nine tests from the CANTAB test battery. Data were analysed using univariate or repeated measures ANCOVA with a between subjects factor of GROUP. Further a priori subgroup analyses were conducted using (1) a two-group factor reflecting DEPENDENCE status and (2) a two-group factor reflecting INJECTING status separately as between subject factors. The homogeneity of variance across groups in repeated-measures design ANCOVAs was assessed by the Mauchly Sphericity Test. NART, age in years, SIMD, total Fagerström score, years in education and past alcohol use in years were used as covariates. A significance level of p<0.01 was applied due to multiple testing, in addition to the post-hoc Bonferroni correction procedure. On the Cambridge Gambling Task (CGT), HEROIN users placed higher bets earlier and risked more. They also showed increased motor impulsivity, impaired strategic planning and visuospatial memory on the Affective Go-NoGo (AGN), Stockings of Cambridge (SOC), and Delayed Matching to Sample(DMS) respectively. METHADONE users deliberated longer and placed higher bets earlier on the CGT, but did not show a tendency to risk more. METHADONE users were also more inattentive and demonstrated poor strategic planning and visuospatial memory on the Spatial Span (SSP) task. The CHRONIC PAIN participants did not exhibit significant impairment in neuropsychological performance on all the CANTAB tasks. Participants from the HEROIN, METHADONE and CHRONIC PAIN groups did not present with impaired cognitive flexibility. Chronic opioid dependence is associated with neuropsychological impairment reflected in altered performance on measures of risk taking and strategic planning. These data support the hypothesis that these neuropsychological impairments reflect an underlying trait vulnerability to drug taking and/or dependence rather than an effect of chronic exposure to opioids. Notably, motor impulsivity and visuo-spatial memory in HEROIN users improved after three weeks stability with methadone. Methadone use seems to confer improvement in some aspects of neuropsychological performance following cessation of heroin and sustains other deficits during long term stable methadone treatment. Dependence and injecting status do not contribute to the causation or deterioration of the identified neuropsychological impairments. Further long term longitudinal studies to help elucidate cognitive endophenotypes responsible for the components in the initiation, continuation and deterioration of neuropsychological deficits present in an opioid dependent population is necessary.